In a fabrication process of a semiconductor device, a photoresist is generally separated and removed from a substrate after its role has been filled in a step of etching or the like. However, such a photoresist film remains unremoved as a permanent film when it is used as a surface-protecting film or a layer insulation film. Therefore, it is required to have electrically and mechanically high characteristics or properties. Further, such a film also must resist a high temperature applied in the fabrication process of the semiconductor device.
In recent years, a film formed of a photosensitive polyimide resin has been used in such an application. The polyimide resin is excellent in electrical and mechanical characteristics, and heat resistance and is used as a surface-protecting film or layer insulation film for a semiconductor device, or the like. In order to selectively provide a film of a polyimide resin on minute parts of the semiconductor device, it is convenient to use a photosensitive polyimide resin.
In recent years, various proposals as to photosensitive polyimide resins the solubility of which varies upon exposure to light have been made. Typical examples of such a photosensitive polyimide resin include compounds with a photopolymerizable acryloyl group introduced into a polyamic acid which is a precursor of a polyimic resin (Japanese Patent Publication Nos. 30207/1980 and 41422/1980.). A precursor of polyimide, in which an acryloyl group has been introduced in the form of a salt, has also be proposed (Japanese Patent Publication No. 52822/1984). In the photosensitive polyimide resin making use of such a polyimide precursor, a film of the photosensitive polyimide resin is formed on a semiconductor device, the film is subjected to patterning exposure to form a pattern, and the polyamic acid is then cyclized by a heat treatment, thereby polyimidating it. At this time, the moiety of the photosensitive group is eliminated and volatilized, thereby reducing the final thickness of the film formed.
The use of the photosensitive polyimide resin permits marked shortening of a pattern-forming process. However, the conventional photosensitive polyimide resins have involved problems such as (1) the synthesizing route is complicated, (2) the photosensitive group is not fully eliminated by polyimidation, and (3) they are insufficient in storage stability and sensitivity to light, and additionally had drawbacks such as (4) a polyimide film formed has a high residual stress, (5) the polyimide film has a high coefficient of thermal expansion, and (6) they are difficult to develop with an alkaline aqueous solution.
The coefficients of thermal expansion of metal wires and various substrates are generally lower than those of resins. When a polyimide film is formed on a substrate, and at this time a difference in coefficient of thermal expansion between the substrate and the polyimide film is great, problems such as cracking, breaking of wire and warpage or curling of the substrate are caused by heating or generation of heat in a production process of a part or upon the use of the part. When the residual stress of the polyimide film is high, similar problems are easy to occur. On the other hand, the development with an organic solvent has involved such problems that developer cost is high, explosion-proof apparatus are required, and human health may be endangered.
There have recently been proposed photosensitive resin compositions comprising an actinic ray-sensitive functional group (Japanese Patent Application Laid-Open Nos. 70661/1992 and 77741/1992). The photosensitive resin compositions disclosed in these publications are prepared by a special process in which a polyamic acid having carbon--carbon double bonds at its terminals is reacted in a solvent composed of an amide compound having a polymerizable carbon--carbon double bond, and are characterized in that the polyamic acid itself is photosensitive and the solvent itself is completely photosensitive. However, the polyamic acids disclosed in these publications are easy to eliminate their modified terminal groups and poor in stability. In these photosensitive resin compositions, the solvent also functions as a photosensitive auxiliary. Therefore, such a composition has involved a problem that when the composition is applied to a substrate and subjected to a heat treatment to form a polyimide film, a residual stress of the polyimide film against the substrate increases to a great extent.
On the other hand, the photosensitive polyimide resin compositions have involved a problem that they are difficult to develop with an alkaline developer or alkaline aqueous solution. For example, the negative type photosensitive polyimide resins disclosed in the above-described Japanese Patent Publication Nos. 30207/1980 and 41422/1980, etc., and those obtained by introducing a photopolymerizable group through an ionic bond into a precursor of polyimide disclosed in Japanese Patent Publication No. 52822/1984 and the like all use organic solvents as their developers. Therefore, such photosensitive polyimide resins have involved such problems that developer cost is high, explosion-proof apparatus are required, and human health may be endangered.
Polyimide precursors heretofore reported as the alkali-developed or alkaline aqueous solution-developed type include, for example, those obtained by introducing a protective group capable of leaving into a carboxyl group or a phenolic hydroxyl group under acid or alkaline conditions [J. Macromol. Sci. Chem., A24(12), pages 1407-1422 (1987), Japanese Patent Application Laid-Open No. 259351/1989 and Japanese Patent Application Laid-Open No. 363361/1992]. However, these precursors require to introduce a substituent group great in steric hindrance through a covalent bond into a main-chain carboxyl group or side-chain hydroxyl group. Therefore, they involve such problems that their synthetic processes become complicated, they require a high curing temperature, and film properties of films formed therefrom are deteriorated.
It has been proposed to incorporate a dihydropyridine derivative, which is a photo-induced base generator, and accelerate the imidation-facilitating effect of a base formed by exposure by a PEB (post-exposure baking), thereby developing a difference in solubility caused by the imidation with an alkaline developer (Japanese Patent Application Laid-Open Nos. 5995/1993 and 281717/1993). However, these processes involves such problems that the PEB treatment must be used, and the film formed has a low sensitivity to exposure, and so an exposure value must be made greater.
It has also been proposed to use an alkaline developer or alkaline aqueous solution as a developer for photosensitive resin compositions with a photopolymerizable monomer added as a crosslinking auxiliary thereto (Japanese Patent Application Laid-Open Nos. 183439/1988 and 100424/1993). In these processes, special crosslinking monomers such as those having an isocyanate structure or a urethane structure are added as the photopolymerizable monomers. Since the base polymers have no photopolymerizable group in their structures, there is a problem that sensitivity and resolution are insufficient.
Most of the photosensitive polyimide resin materials known to date have a coefficient of thermal expansion higher than 30 ppm/.degree. C. in terms of a polyimide film formed by patterning on a substrate. Therefore, the coefficient of thermal expansion greatly differs from that of the substrate (in particular, a metal substrate having a coefficient of thermal expansion of 20 ppm/.degree. C. or lower), which causes problems such as separation of the polyimide film from the substrate, distortion of a pattern form and warpage of the substrate. Therefore, such photosensitive polyimide resin materials have involved a problem of reliability from the viewpoint of minute processing, and their applications have also been limited. Besides, the conventional photosensitive polyimide resin materials have been developed mainly with an organic solvent, and it has thus been desired to develop a photosensitive resin material developable with an aqueous developer.